Over the last decade, there has been significant developments in the area of solid state laser systems which are optically pumped using a laser diode. Laser diodes are desirable as pump sources since they are compact and do not create the heating problems encountered with other light sources such as flashlamps. The light from the laser diode can also be focused allowing for relatively efficient longitudinal or end pumping configurations.
There are also some drawbacks to using laser diodes. One significant problem relates to the fact that laser diodes (and particularly high power laser diodes) have an emitter region with a linear geometry and produce an asymmetric output wherein the width is much greater than the height. In a diode pumped laser system, maximum efficiency is achieved when the mode volume of the laser diode is matched well with the circulating mode of the laser resonator. Ideally, the circulating mode of the beam is relatively symmetric so that a TEM.sub.00 or fundamental mode output can be generated. Significant efforts are therefore required in order to modify the highly asymmetric laser diode output beam in order to match the symmetric resonator beam.
In practice, most efforts at modifying the asymmetric output of the laser diode are directed towards improving the "spatial" symmetry of the beam. Using various optical elements such as prisms and lenses, the aspect ratio of the beam can be modified. Unfortunately, spatial asymmetry is only part of the problem. More specifically, broad area emitters also have severe "brightness" or "radiance" asymmetry. Brightness is defined as the number of photons per area per unit solid angle and is inversely proportional to the product of the area times the divergence angle. In a typical high power laser diode, the divergence in the narrow, height dimension is greater than the divergence in the width dimension. However, since the width dimension is so much greater than the height dimension, the brightness in the height dimension can still be a hundred times greater than in the width dimension. In order to maximize the pump efficiency, it would be desirable to minimize the brightness asymmetry.
Unfortunately, these asymmetries in brightness are not modified with the basic optics used to correct for spatial asymmetries. More specifically, when a basic optical system is used to reduce the width of the beam, the divergence will proportionally increase. As can be appreciated, since the product of the width and divergence remains the same, brightness will not be effected. Therefore, there is a need for an improved optical system which can not only modify the spatial asymmetries but modify the brightness asymmetries as well so that higher power laser diodes can be used to pump lasers.
Accordingly, it is an object of the subject invention to provide an optical system for improving the brightness symmetry of the output of a broad area laser diode or laser diode bar.
It is another object of the subject invention to provide an optical system for improving the mode quality symmetry of the output of a broad area laser diode.
It is a further object of the subject invention to provide an optical system for improving the symmetry of the output of a broad area emitter so that it can be more efficiently coupled into the end of a symmetric target such as a gain medium or an optical fiber.
It is still another object of the subject invention to provide a diode laser pumped solid state laser capable of generating a higher output power.